The present invention relates generally to an optical pickup device for an optical information recording and/or reproducing apparatus using an optical recording medium such as optical disk, magneto-optical disk or the like.
Generally, a conventional optical pickup device of an optical information recording and/or reproducing apparatus comprises an objective lens which acts to converge a laser beam emitted from a semiconductor laser onto an optical recording disk so as to form a very small beam spot on an information recording track of the optical recording disk. In order to correct focus and tracking errors of the objective lens in relation to the optical recording medium, the objective lens is so constructed as to be slightly moved by a bidirectional actuator in a first direction perpendicular to the information track of the optical recording disk and a second which is normal to the disk surface. In the conventional optical pickup device of this type, generally, the reflected (or return) beam from the optical recording disk passes through the objective lens, quarter-wave plate, and polarized beam splitter. On the return trip, the beam is split by a beam splitter into two parts, one being used as a tracking error signal for the detection of tracking error and the other being used as a focus error signal for the detection of focus error.
As a method for detecting the tracking error of the laser beam converged by the objective lens in relation to the information track of the optical recording disk, there has been well known a push-pull method in which a set of two photodiodes is used for comparing the amount of laser beams incident on the respective photodiodes. The two photodiodes are separated from each other through a separation line.
On the other hand, as a method for detecting the focus error of the objective lens, there has been well known an astigmatic method in which a set of four photodiodes is used with convex and cylindrical lenses, for comparing the amount of incident beam on the respective photodiodes. The four photodiodes are separated from each other through separation lines. An information signal can be obtained by at least either of the above-mentioned two sets of the photodiodes.
However, generally, the push-pull method has an disadvantage in that an error is apt to occur in the tracking error signal due to a generation of deviation of the optical axis of the objective lens from the separation line between the two photodiodes.
In order to reduce the above-mentioned error in the tracking error signal, the conventional optical pickup device has been provided with a light emitting diode, which is secured to the objective lens, and an additional photodiode for detecting the light emitted from the light emitting diode. Namely, the additional photodiode is adapted to produce a lens position detection signal when the optical axis of the objective lens is deviated from the separation line between the two photodiodes is detected, and the actuator for the objective lens acts to minimize the deviation of the optical axis of the objective lens in accordance with the lens position detection signal.
However, the above-mentioned light emitting diode and additional photodiode causes the optical pickup device to be increased in size and complicated in construction. Particularly, the light emitting diode secured to the objective lens causes the load of the actuator for the objective lens to be increased, resulting in a large-sized and complicated objective driving system.